Gain control circuits



Nov. 14, 1939. R. R. BLAIR GAIN CONTROL CIRCUITS Filed Sept. 8, 1938 HEATER CURRENT I... Ho 9 SHUNT F/LTER /4 EDANCE SERIES IMPEDANCE e INVENTOR 4 RR. BLAIR 2 227, flilefl O Rf: INPUT IN 03.

A TTORNE V Patented Nov. 14, 1939 UNITED STATES PATENT OFFICE Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application September 8; 1938, Serial No. 228,999

14 Claims.

This invention relates to regulator circuits and particularly to circuits for efiecting automatic regulation of the gain on signal transmission systems.

One object of the invention is to provide regulator circuits having an oscillator operating on a steeply inclined portion of its characteristic curve for maintaining a characteristic of a controlled circuit substantially constant.-

Another object of the invention is to provide a signal transmission line with gain control circuits having an oscillator operating on a steeply inclined. cut-off portion of its characteristic curve that shall control the gain of an amplifier on the transmission line according to the oscillator output for maintaining the amplifier output substantially constant.

A further object of the invention is to provid a signal transmission line having an amplifier therein With gain control circuits of the aboveindicated character that shall control the beta circuit of the amplifier in an improved manner to maintain the amplifier output constant.

In long signal transmission lines it is necessary to provide amplifiers or repeaters at intervals along the line to maintain the energy level of the signals above a predetermined level. Regulater or gain control circuits are required at certain of the repeater stations tohold the energy level of the signals substantially constant. The regulator or gain control circuits may be controlled in accordance with the energy level of the signals or in accordance with the energy level of a pilot current. v

This invention has been illustrated by means of a system transmitting high frequency carrier current signals. A pilot current is transmitted with the carrier currents for governing the gain on the transmission system.

According to the system employed todescribe the invention, a feedback amplifier of the type disclosed in the patent to H. S. Black No. 2,102,671, December 21, 1937, is controlled to maintain the energy level of signals on the transmission line substantially constant. In one form of the invention a resistance element having a high negative temperature coefficient of resistance is connected across the beta circuit of the amplifier. The resistance element is heated according to the energy level of the pilot current on the transmission line for so governing the amplifier as to maintain the output thereof substantially'constant. The resistance element in the beta circuit of the amplifier is preferably composed of silver sulphide but maybe comcontrol the heating of the resistance element in the beta circuit of the amplifier are connected to the transmission line beyond the amplifier therein. The gain control circuits comprise a filter connected to the transmission line for selecting the pilot current. of the filter is connected tothe input circuit of a space discharge device. The space discharge device is preferably in the form of a pentode tube and amplifies the selected pilot current. The output circuit of the pentode tube is tuned to the frequency of the pilot current. The output from the pentode tube is rectified for supplying a negativebias to be impressed on the grid of a low frequency space discharge oscillator. The oscillator, which is controlled by the rectified pilot current, has an input-output characteristic curve With a steeply inclined cut-01f portion. The oscillator is operated on the steeply inclined portion of its characteristic curve so that a small change in the energy level of the pilot current causes a great change in the output of the oscillater. The output of the oscillator is employed for heating a resistance element connected across the beta circuit of the line amplifier.

An increase in the energy level of the pilot current on the transmission line Will cause an increase in the negative bias impressed on the grid of the low frequency oscillator. A small change in the negative bias on the low frequency oscillator causes a more than proportional change in the output from the oscillator. The change in output from the oscillator causes a large change in the resistance connected across the beta circuit to control the line amplifier and reduce the energy level of the signals on the transmission oscillator in place of increasing the negative bias. 9

Thus, in the modification an increase in the energy level of the pilot current Would cause a decrease in the negative bias on ,the grid of the oscillator to reduce the output from the oscillator.

In the accompanying drawing,

The output circuit the resist- Fig. 1 is a diagrammatic view of regulator circuits constructed in accordance with the invention;

Fig. 2 is a diagrammatic view of a modification of the circuits shown in Fig. 1;

Fig. 3 is a curve diagrammatically illustrating the characteristic curves of the regulator circuits shown in Figs. 1 and 2.

Referring to Fig. 1 of the drawing, an amplifier I of the feedback type is shown in a transmission line having input conductors 2 and 3 and output conductors 4 and 5. The amplifier I may be of the type disclosed in the patent to H. S. Black No. 2,102,671, December 21, 1937. The amplifier I is shown comprising three tubes 6, l, and 8 which may be of any suitable type. An impedance pad I0 is shown connected across the beta circuit 9 of the amplifier I. One element of the impedance pad II] comprises a resistance element I I having a high negative temperature coefficient of resistance. The resistance element I I is controlled by a heater element I2. An increase in the current fiow through the heater element I2 decreases the resistance of the resistance element II for decreasing the feedback through the beta circuit 9 to increase the gain of the amplifier. A decrease in the current flow through the heater coil I2 produces an opposite action.

Regulator circuits I3 are provided for controlling the heater element I2 to maintain the output from the amplifier I substantially constant. The transmission line transmits not only carrier currents, but also a pilot current. The pilot current is employed for controlling the operation of the regulator or gain control circuits I3. The transmission line may be a coaxial cable or a cable carrying carrier frequency currents. The control circuits I3 comprise a filter I4 which is preferably of the crystal type. The filter selects the pilot current which controls the operation of the regulator circuits. The output from the filter I4 is connected to a tuned radio frequency pentode amplifier I 5. The amplified radio frequency output from the pentode I5 is rectified by a space discharge device I6 for controlling the amplitude of the output from a low frequency oscillator H. The output from the oscillator I I is connected by a transformer I8 to the heating element I2.

The pentode amplifier I5 comprises an anode I9, a cathode 20, a control grid 2I, a suppressor grid 22 and a screen grid 23. Plate potential for the pentode I5 is supplied by a battery 24 through a potential divider comprising resistance elements 25 and 26. The plate circuit for the pentode tube may be traced from one terminal of the battery 24 through resistance element 26, inductance 21 shunted by a condenser 28, anode I9, cathode 20, resistance elements 29 and 30 and ground return to the other terminal of the battery 24. The gain of the pentode tube I5 is controlled by varying the resistance element 30. The resistance element 29 prevents the possibility of reducing the bias on the pentode I5 to zero. A resistance 3I a is connected across the input circuit of the pentode I5 to provide a grid return path when the circuit of the filter I4 is not completed. The resistance elements 25 and 26 perform the function of voltage dividers to reduce the potential impressed on the anode I9 and the screen grid 23. The inductance 21' and the cap ty 28 serve to tune the amplifier I5 to the frequency of the selected pilot current. Three condensers 3| 32 and 33 serve as by-passing condensers in the operation of the pentode amplifier I5.

The rectifier I6 which is preferably of the space discharge type comprises an anode 34 and a cathode 35. The direct current component obtained by the rectifier I6 from the output of the amplifier I5 appears across a resistance element 36. A resistance element 31 in combination with a capacitance 38 serve as a low-pass filter. The radio frequency output circuit from the amplifier I5 may be traced from the anode I9 through a condenser 39, rectifier I6 and condenser 32 to the cathode 20.

The oscillator space discharge device I! comprises an anode 40, a cathode 4|, a grid 42. The oscillator I1 is an audio frequency oscillator and preferably oscillates at approximately 2500 cycles per second. The transformer I8 which couples the oscillator I! to the heater coil I2 also provides the necessary grid excitation for the oscillator I! through a condenser 43. An inductance 44 in the secondary circuit of the transformer I8 is a radio frequency choke which may be a part of a cross-talk filter in the feedback amplifier I. Negative bias for the grid 42 of the oscillator Il may be transmitted through a grid leak resistance 45.

The oscillator I! is adjusted to have an outputinput characteristic curve with a sharply inclined cut-off portion. The oscillator is operated along the sharply inclined cut-off portion of its characteristic curve so that a small change in bias on the grid 42 of the oscillator causes a more than proportional change in the output from the oscillator.

In Fig. 3 of the drawing the curve A shows the characteristic curve of the regulator circuit shown in Fig. 1. The abscissae represent the decibels input to the amplifier I5 and the ordinates represent the output from the oscillator I! which is supplied to the heater coil I2. The output from the oscillator I7 is controlled by means of an adjustable resistance 46. The condenser 41 serves as a by-pass condenser. The oscillator I'I operates with an input-output characteristic curve having a steeply inclined cut-off portion similar to the steeply inclined portion of the curve A shown in Fig. 3 of the drawing.

If the output signals from the repeater or amplifier I tend to increase, the energy level of the pilot current will tend to increase. An increase in the energy level of the pilot current will increase the potential appearing across the resistance element 36. The potential across the resistance 36 represents the rectified output from the pentode amplifier I 5. An increase in the potential across the resistance element 36 increases the negative bias impressed on the grid 42 of the oscillator II. Increase in the negative potential on the grid of the oscillator reduces the output from the oscillator to reduce the current fiow through the heater element I2. This reduces the temperature of the heat element II, which is preferably composed of silver sulphide. The resistance of the element II is increased to increase the feedback through the beta circuit 9 from the amplifier I. This reduces the gain of the amplifier.

If the energy level of the signals beyond the amplifier I falls below normal value the reduced energy level of the pilot current which is selected by the filter I4 and amplified by the amplifier I5 produces a lower potential drop across the resistance element 36. The negative bias on the grid 42 of the oscillator I1 is reduced to increase the current flow through the heater element I2.

An increase in the current flow from the heater element l2 increases the temperature of'the resistance element II to decrease the resistance thereof. The feedback on the amplifier l is thereby reduced to increase the gain by the amplifier I. In the above-described manner the gain of the amplifier l is controlled to maintain the amplifier output substantially constant.

Referring to Fig. 2 of the drawing a modification of'the invention is shown wherein the beta circuit'of a line amplifier is controlled by means of a resistance element connected in series therewith. Like parts in the circuit shown in Fig. 2 to those shown in Fig. 1 of the drawing will be indicated by similar referencecharacters. The feed-back amplifier l in the transmission line is connected to input conductors l. and 3 and output conductors 4 and 5. The beta circuit 9 is shown having a series impedance network 56 and shunt impedance networks 5| and 52. A portion of the series impedance network comprises a resistance element 53 which is similar to the resistance element ll shown in Fig. 1 of the drawing and preferably is composed of silver sulphide. The resistance element 53 is controlled by a heater element 54. The heater element 54 is controlled by regulator circuits 55 in accordance with the energy level of a pilot current on the output conductors 4 and 5.

A filter m which is similar to the filter l4 shown in Fig. l of the drawing selects a pilot current and impresses the same on a space discharge device in the form of a tuned pentodediode 56. The pentode portion of the device 56 amplifies the radio frequency pilot current. The diode portion of the device 56 rectifies the amplified pilot current to supply rectified potential. for controlling an oscillator device 5l. The outputfrom the oscillator 57 controls the current flow through the heater element 54.

The pentode portion of the device 56 comprises an anode 56, a cathode 59, a control grid 69, a suppressor grid 6| and a screen grid 62. Anode potential for the pentode of the device 56 is supplied from a suitable source of alternating current 63. The anode circuit may be traced from one terminal of the source 63 through an inductance 64, the anode 58, the cathode 59, and resistance elements 65, 66 and 6'! to the other terminal of the source 63. Biasing potential for the control grid 66 is obtained by the potential drop across the resistance element '65. Initial negative bias for the control grid 66 of the oscillator device 51 is obtained fro-m the potential drops across the resistance elements 65, 66 and 61. This initial negative bias on the grid of the oscillator device 51 is at least large enough to prevent the device from oscillating, Radio frequency signals must be impressed upon the amplifier 56 before oscillation of the device 57 can take place as will be hereinafter explained.

The inductance coil 64 in combination with an inductance coil 66 are tuned by an adjustable condenser 16 to the frequency of the pilot current admitted by the filter l4. Acondenser ll serves as a blocking condenser. The radio frequency output circuit from the pentode portion of the device 56 may be traced from the-condenser Hl shunted by inductance 69 through plate 12 of the diode portion of the device 56, cathode 56, condenser 13 and grounded return to the other terminal of the condenser 10 shunted by the inductance element 69. The rectified current obtained from the diode portion of the device 56 appears across the resistance element l4. The

drop across theresistance element 14 subtracts from the initial negative bias upon the grid 68 of the oscillator device 57. A source of alternatto the oscillator 51 by the potential dropacross the resistance elements 65, 66 and 61, and third. a diode detector for rectifying the radio frequency output of the pentode portion of the device; A condenser 18 serves to filter the rectifiedoutp-ut of the device 56 appearingacross the resistance elements 65, 66 and 61. A-filter comprising resistance element 86 and capacity elements 13, 8| and 86 is provided for removing any -cycle modulation products that may be in the biasing circuitfor the'oscillator 51. denser 8! serves as a filter condenser for the bias on device 56 and condensers 88, I96, l6! and i6 serve as by-pass condensers.

' The oscillator device 51 comprises an anode 89, a cathode 16, the control grid 68 and a screen grid 90. The output from the oscillator 51 is connected by a transformer 9| to the heating coil 56. The bias on the control grid 68 is governed according to the negative potential across the resistance elements 65,66 and 61 opposed by the positive potential drop across the resistance element 14. The positive potential drop across the resistance element it varies according to the energy level of the pilot current of the transmission line. The grid circuit for the oscillator 51 may be traced from one terminal of the resistance element 61 through resistance element 86, resistance element 19, grid 68, cathode l6, inductance element 69, resistance element 14 and resistance elements and 66 to the other terminal of the resistance element 67. The transformer 9i and a condenser 92 connected across the. primary winding of the transformer determine the frequency at which the oscillator 5'! oscillates. The

condenser 86 provides the necessary feedback path r from the secondary winding of the transformer to the control grid 68. The maximum output of the oscillator 51 is controlled by the screen grid potential obtained from the potential drop across the resistance element 93. The resistance element 93 in series with a resistance element 94 is connected across a battery 95. Oscillator outputs from zero to any given maximum are obtained by varying the bias on the control grid 68 through the associated grid leak resistance 19. The radio frequency input voltage at which the oscillator cuts off is controlled by varying the bias on the oscillator 51 by means of the resistance elements 66 and 61. A fine adjustment may be made by the resistance element 66 and a coarse adjustment may be made by means of the resistance element 61. v

If the signals on the output conductors 4 and 5 are above normal value, the pilot current selected by the filter M will have an amplitude above normal value. The pilot current'amplified by the pentode portion of the device 56 will be rectified by the diode portion of the device 56 and impressed on the resistance element 14. Increasing the positive potential. bias obtained from the Con.-

.drop across the resistance element 14 increases the output from the oscillator device 51. As before set forth, negative bias is impressed on the grid 68 of the oscillator by the potential drop across resistance elements 65, 66 and 61. A positive bias in opposition to negative bias will be obtained by the potential drop across the resistance element 14. An increase in the oscillator output increases current flow through the heater element 54. This increases the temperature of the resistance element 53 and lowers the resistance thereof. Lowering the resistance in the beta circuit 9 increases feedback and lowers the amplifier output to bring the signal energy level to normal.

In case the signals on the output conductors 4 and 5 fall below normal value the amplified rectified pilot current impressed on the resistance element 74 is reduced to reduce the output from the oscillator 51. A reduction in the output from the oscillator 51 reduces the current flow through the heater element 54. This reduces the temperature of the resistance element 53 and increases the resistance thereof. This reduces the feedback of the amplifier I to increase the output therefrom. In the above-described manner the output from the amplifier l is maintained substantially constant.

The curve B shown in Fig. 3 of the drawing illustrates the characteristic of the regulator circuit shown in Fig. 2 of the drawing. The abscissae represent decibels input to the amplifier 56 and the ordinates represent the output from the oscillator which is supplied to the heater coil 54. The oscillator 51 operates with an input-output characteristic curve having a steeply inclined cut-ofi portion similar to the steeply inclined portion of the characteristic curve B shown in Fig. 3 for the regulator circuits illustrated in Fig. 2 of the drawing. The oscillator 51 operates on the steeply inclined cut-off portion of the characteristic curve. The oscillator 51 shown in Fig. 2 of the drawing, the same as the oscillator l1 shown inFig. 1 of the drawing, serves to efiect an output which is more than proportional to any change in the pilot current on the transmission line.

Modifications in the circuits and in the arrangement and location of parts may be made within the spirit and scope of the invention. Such modifications are intended to be covered by the appended claims.

What is claimed is:

1. A controlled circuit having a characteristic thereof maintained substantially constant, a space discharge oscillator having a control grid, means for impressing a variable potential on said grid according to the characteristic of said circuit, means for operating said oscillator on a steeply inclined portion of the input-output characteristic curve thereof when controlled by the potential impressed on the grid, and means controlled according to the output from said oscillator for maintaining the characteristic of the controlled circuit substantially constant.

2. A transmission line carrying signal and pilot currents and having an amplifier therein, a space discharge oscillator having a control grid, means for governing the potential on said grid according to the energy level of said pilot current, means for operating the oscillator on a steeply inclined portion of the input-output characteristic curve thereof when controlled according to the energy level of the pilot current, and means for controlling said amplifier according to the output from said oscillator.

3. A signal transmission line having an amplifier therein, a space discharge oscillator having a control grid, said oscillator having an inputoutput characteristic curve with a portion thereof which effects a more than proportional change in the output for a change in the input, means for impressing potential on said grid according to the energy level of the signals on the line for controlling said oscillator, means for operating said oscillator to effect a more than proportional change in the oscillator output upon signal energy change when controlled according to the energy level of the signals on the line, and means governed according to the oscillator output for controlling said amplifier to maintain substantially constant amplifier output.

4. A signal transmission line, an amplifier in said transmission line having a beta circuit, a space discharge oscillator having a control grid, means for impressing varying potential on said grid according to the energy level of the signals on said line, means for adjusting said oscillator to have an input-output characteristic curve with a steeply inclined cut-oif portion and for operating the oscillator on the steeply inclined portion of the characteristic curve when controlled by the varying potential impressed on the grid, and means for controlling said beta circuit according to the output from said oscillator to maintain the amplifier output substantially constant.

5. A transmission line carrying signal and pilot currents and having an amplifier therein, means comprising a resistance element having a high negative temperature coefficient of resistance for controlling said amplifier, a heater element for controlling said resistance element, a space discharge oscillator having a control grid, said oscillator having an input-output characteristic curve with a portion thereof which effects a more than proportional change in the output for a change in the input, means for impressing potential on said grid according to.

the energy level of said pilot current for controlling said oscillator, means for operating said oscillator to effect a more than proportional change in the oscillator output when controlled according to pilot current change, and means governed according to the oscillator output for controlling said heater element to maintain the amplifier output substantially constant.

6. A transmission line carrying signal and pilot currents, an amplifier having a beta circuit in said line, means comprising a resistance element in said beta circuit for controlling the amplifier, said resistance element having a high temperature coeflicient of resistance, a heater element for controlling said resistance element, a space discharge oscillator having a control grid, means for impressing potential on said grid which varies according to the energy level of said pilot current on the line beyond said amplifier, means for adjusting said oscillator to have an input-output characteristic curve with a steeply inclined cut-off portion and for operating the oscillator on the steeply inclined portion of the characteristic curve when controlled by the varying potential impressed on the grid, and means for controlling said resistance element by said heater element according to the output from said oscillator to maintain the amplifier output substantially constant.

7. A transmission line carrying signal and pilot currents, an amplifier in said line, a space discharge oscillator having a control grid, means for impressing negative potential on said control grid which. varies according to the energy level of said pilot current, means for adjusting said oscillator to have an input-output characteristic curve with a steeply inclined cut-off portion and for operating the oscillator on the steeply inclined portion of the characteristic curve, and means for controlling said amplifier according to the output from said oscillator to maintain the amplifier output substantially constant.

8. A transmission line carrying signal and pilot currents, an amplifier in said line, a space discharge oscillator having a control grid, means for filtering said pilotcurrent from said line beyond the amplifier and for amplifying and rectifying the filtered pilot current to impress negative potential on the grid of said oscillator, means for adjusting said oscillator to have an input-output characteristic curve with a steeply inclined cut-off portion and for operating the oscillator on the steeplyinclined portion of the characteristic curve, and means for controlling said amplifier according to the output from said oscillator.

9. A signal transmission line, an amplifier in said line having a beta circuit, means comprising a resistance element connected across said beta circuit for controlling said amplifier, said resistance element having a high negative temperature coefficient of resistance, a heater element for controlling said resistance element, a space discharge oscillator having a control grid, means for impressing negative potential on said grid which varies according to the energy level of the signals on said line, means for adjusting said oscillator to have an input-output characteristic curve with a steeply inclined cut-off portion and for operating the oscillator on the steeply inclined portion of the characteristic curve, and means for controlling said heater element according to the output from said oscillator to maintain the amplifier output sub stantially constant.

10. A transmission line carrying signal and pilot currents of radio frequency, an amplifier in said line, a filter connected to said line beyond said amplifier for selecting said pilot frequency, an amplifier tuned to said pilot frequency and connected to said filter, a space discharge oscillator having a control grid, means for rectifymaintain the line amplifier output substantially constant.

11. A signal transmission line, an amplifier in saidline having a beta circuit, means comprising a resistance element series connected in said beta circuit for controlling the amplifier, said resistance element having a high negative temperature coefficient of resistance, a heater element for controlling said resistance element, a space discharge oscillator having a control grid, means for impressing less negative potential on said grid which varies according to the energy level of the signals on said line, means for adjusting said oscillator to have an input-output characteristic curve with a steeply inclined cutoff portion and for operating the oscillator on the steeply inclined portion of the characteristic curve, and means for controlling said heater cle ment according to the output from said oscillator to maintain the amplifier output substantially constant.

12. A transmission line carrying signal and pilot currents, an amplifier in said line, a space discharge oscillator having a control grid, means for filtering said pilot current from the line H beyond said amplifier and for amplifying and rectifying the filtered pilot current to impress less negative potential on the grid of said oscillator, means for adjusting said oscillator to have an input-output characteristic curve with a steeply inclined cut-01f portion and for operating the oscillator on the steeply inclined portion of the characteristic curve, and means for controlling said amplifier according to the output from said oscillator to maintain the amplifier output substantially constant.

13. A transmission line carrying signal and pilot currents, an amplifier in said line, a space discharge oscillator having a control grid, means for impressing a negative bias on said control grid, means for impressing a less negative potential on said control grid which varies according to the energy level of said pilot current, means for adjusting said oscillator to have an input-output characteristic curve with a steeply inclined cut-off portion and for operating the oscillator on the steeply inclined portion of the characteristic curve, and means for controlling said amplifier according to the output from said oscillator to maintain the amplifier output substantially constant.

14. A transmission line carrying signal and pilot currents, an amplifier in said line, a space discharge oscillator having a control and a screen grid, means for impressing potential on said screen grid to limit the maximum output of said oscillator device, a space discharge device comprising diode and amplifier portions, means for impressing alternating space potential on the amplifier portion of said device, means for filtering the pilot current on said line and for impressing the selected pilot current on the input circuit of the amplifier portion of said device, means for impressing negative potential from the plate circuit of said device on the control grid of said oscillator, means for rectifying the output from amplifier portion of said device by the diode portion thereof to impress a variable less negative potential on the control grid of said oscillator, and means for controlling said line amplifier according to the output from said oscillator.

ROYER R. BLAIR. 

